You should include instructions, the ability to pick up items, better graphics, and improve the melee attack.
NO HEALTH BAR....
It honestly took me a while to figure out that you are unable to move left or right unless you close the first box. The attack doesn't seem to work, therefore I am having a very rough time "killing tons of aliens".
The game is good, but needs some polish on. Try to fix the camera, it's very jumpy. The melee doesn't work very well and the aliens take a lot of damage to destroy. Otherwise the core elements and the idea is good.
Biggest complaint is the inability to pick up items, and the fact that you get stuck on the little blades of grass on the first level, and the music seems very out of place for a game like this and cuts off abruptly at certain points. All around it doesn't seem good at all.